Field of the Invention
This application is directed at providing correction for astigmatism, including provision of systems and methods that use parameters that were previously not systematically accounted for to improve patient outcomes.
Description of the Related Art
Ophthalmic lenses, such as spectacles, contact lenses and intraocular lenses, may be configured to provide both spherical and cylinder power. The cylinder power of a lens is used to correct the rotational asymmetric aberration of astigmatism of the cornea or eye, since astigmatism cannot be corrected by adjusting the spherical power of the lens alone. Lenses that are configured to correct astigmatism are commonly referred to as toric lenses. As used herein, a toric lens is characterized by a base spherical power (which may be positive, negative, or zero) and a cylinder power that is added to the base spherical power of the lens for correcting astigmatism of the eye.
Toric lenses typically have at least one surface that can be described by an asymmetric toric shape having two different curvature values in two orthogonal axes, wherein the toric lens is characterized by a “low power meridian” with a constant power equal to the base spherical power and an orthogonal “high power meridian” with a constant power equal to the base spherical power plus the cylinder power of the lens. Intraocular lenses, which are used to replace or supplement the natural lens of an eye, may also be configured to have a cylinder power for reducing or correcting astigmatism of the cornea or eye.
Several pervasive problems arise in the selection of the proper toric lens configuration. These problems relate to the need to provide the correct lens configuration for the eye as it will exist after surgery. First, conventional selection is based on pre-operative anterior corneal keratometry. That is, the anterior surface of the cornea is measured before surgery to determine the curvature in orthogonal (e.g., horizontal and vertical) meridians and the toric lens configuration is selected primarily based upon this measurement. These measurements do not conventionally include measurements or estimates of the curvature of the posterior surface of the cornea, which can have a significant impact on a patient's overall astigmatism. Failure to account for the posterior corneal astigmatism can contribute to improper selection of toric lens configuration, which can require further correction.
Another problem arises from surgical steps taken after standard anterior corneal keratometry. That is, surgery can induce or exacerbate astigmatism. In practice, an incision is made at one location of the eye prior to introducing an intraocular lens into the eye. This incision changes the properties of the cornea. The changes can include steepening or flattening of the cornea along a meridian. If the incision flattens an already lower curvature meridian, the astigmatism also can be increased. Failure to accurately and systematically account for the contribution of this surgically induced astigmatism can lead to sub-optimal outcomes.
Astigmatism is sometime characterized as “against-the-rule” or “with-the-rule”. FIG. 1 shows two meridians that may be found to have different curvatures in a cornea with astigmatism. The meridian A is a vertical meridian of the anterior surface of the cornea and the meridian B is a horizontal meridian of anterior surface of the cornea. If the curvature of vertical meridian A is steeper than that of horizontal meridian B, the eye is said to have “with-the-rule” astigmatism, as depicted in FIG. 1a. If the curvature of horizontal meridian B is steeper than that of vertical meridian A, the eye is said to have “against-the-rule” astigmatism, as depicted in FIG. 1b. While not always the case, typically the steep meridian of the anterior corneal surface tends to change from vertical to horizontal with increasing age, while that of the posterior corneal surface tends to retain its vertically steep alignment. Thus, posterior corneal astigmatism generally contributes to against-the-rule astigmatism. See, Douglas D. Koch et. al. “Contribution of posterior corneal astigmatism to total corneal astigmatism,” J Cataract Refract Surg 2012; 38:2080-2087.
As discussed above, surgically induced astigmatism (SIA) can affect both the magnitude and direction of the principal astigmatic meridians of the cornea. Conventional methods include contribution from SIA based on an input diopter value provided by a physician at an incision location also provided by the physician. However, interactions between incision location and the orientation of the steep meridian in determining surgically induced astigmatism (SIA) are not known or conventionally part of toric IOL selection. Thus, although conventional methods do account for SIA, they do so in an un-controlled manner.
In view of the above discussed unknowns, surgeons have adopted a few “rules of thumb” when selecting an appropriate intraocular lens for implantation in a patient. For example, one rule of thumb is to over-correct against-the-rule astigmatism and to under-correct with-the rule astigmatism. Although, the rules of thumb may provide a satisfactory post-operative refractive outcome for some patients, many patients require additional correction (e.g., eyeglasses) after surgery due to the conventional inexact techniques. Even for the patients that have acceptable outcomes, the use of these rules of thumb complicates IOL selection for the physician. Accordingly, it would be desirable to have a method that can more precisely predict the post-surgical refractive outcome for most patients.